linux-kernel - Re: [PATCH 2/3] HID: logitech-hidpp: Fix double free on managed resource

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Message-ID: <20220423150212.GA6661@elementary>
Date:   Sat, 23 Apr 2022 17:02:12 +0200
From:   José Expósito <jose.exposito89@...il.com>
To:     Hans de Goede <hdegoede@...hat.com>
Cc:     hadess@...ess.net, dmitry.torokhov@...il.com, rydberg@...math.org,
        lains@...eup.net, jikos@...nel.org, benjamin.tissoires@...hat.com,
        linux-input@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] HID: logitech-hidpp: Fix double free on managed
 resource

On Sat, Apr 23, 2022 at 01:41:30PM +0200, Hans de Goede wrote:
> Hi,
> 
> On 4/22/22 18:17, José Expósito wrote:
> > As described in the documentation for devm_input_allocate_device():
> > 
> >   Managed input devices do not need to be explicitly unregistered or
> >   freed as it will be done automatically when owner device unbinds from
> >   its driver (or binding fails).
> > 
> > However this driver was explicitly freeing the input device, allocated
> > using devm_input_allocate_device() through hidpp_allocate_input().
> > 
> > Remove the call to input_free_device() to avoid a possible double free
> > error.
> 
> Actually calling input_free_device() on a devm allocated input device
> is fine. The input subsystem has chosen to not have a
> separate devm_input_free_device(), instead input_free_device() knows
> if a device is allocated through devm and then also frees the devres
> tied to it:
> 
> void input_free_device(struct input_dev *dev)
> {
>         if (dev) {
>                 if (dev->devres_managed)
>                         WARN_ON(devres_destroy(dev->dev.parent,
>                                                 devm_input_device_release,
>                                                 devm_input_device_match,
>                                                 dev));
>                 input_put_device(dev);
>         }
> }

Hi Hans, 

Thanks for the code review.

Obviously, I completely misunderstood these functions, my bad.
Thanks for the explanation.

Please ignore the patchset.

Jose

> > 
> > Fixes: c39e3d5fc9dd3 ("HID: logitech-hidpp: late bind the input device on wireless connection")
> > Signed-off-by: José Expósito <jose.exposito89@...il.com>
> > ---
> >  drivers/hid/hid-logitech-hidpp.c | 6 +-----
> >  1 file changed, 1 insertion(+), 5 deletions(-)
> > 
> > diff --git a/drivers/hid/hid-logitech-hidpp.c b/drivers/hid/hid-logitech-hidpp.c
> > index 81de88ab2ecc..9c00a781ab57 100644
> > --- a/drivers/hid/hid-logitech-hidpp.c
> > +++ b/drivers/hid/hid-logitech-hidpp.c
> > @@ -3957,11 +3957,7 @@ static void hidpp_connect_event(struct hidpp_device *hidpp)
> >  	}
> >  
> >  	hidpp_populate_input(hidpp, input);
> > -
> > -	ret = input_register_device(input);
> > -	if (ret)
> > -		input_free_device(input);
> > -
> 
> The original code does look wrong there though, since the input device
> is free-ed it should not be stored in hidpp->delayed_input, so this should be comes:
> 
> 	ret = input_register_device(input);
> 	if (ret) {
> 		input_free_device(input);
> 		return;
> 	}
> 
> 
> Regards,
> 
> Hans
> 
> 
> >  	hidpp->delayed_input = input;
> >  }
> >  
> 
>